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<a name="Machine-Code"></a>
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Previous: <a href="Source-Path.html#Source-Path" accesskey="p" rel="prev">Source Path</a>, Up: <a href="Source.html#Source" accesskey="u" rel="up">Source</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
<a name="Source-and-Machine-Code"></a>
<h3 class="section">9.6 Source and Machine Code</h3>
<a name="index-source-line-and-its-code-address"></a>

<p>You can use the command <code>info line</code> to map source lines to program
addresses (and vice versa), and the command <code>disassemble</code> to display
a range of addresses as machine instructions.  You can use the command
<code>set disassemble-next-line</code> to set whether to disassemble next
source line when execution stops.  When run under <small>GNU</small> Emacs
mode, the <code>info line</code> command causes the arrow to point to the
line specified.  Also, <code>info line</code> prints addresses in symbolic form as
well as hex.
</p>
<dl compact="compact">
<dd><a name="index-info-line"></a>
</dd>
<dt><code>info line <var>location</var></code></dt>
<dd><p>Print the starting and ending addresses of the compiled code for
source line <var>location</var>.  You can specify source lines in any of
the ways documented in <a href="Specify-Location.html#Specify-Location">Specify Location</a>.
</p></dd>
</dl>

<p>For example, we can use <code>info line</code> to discover the location of
the object code for the first line of function
<code>m4_changequote</code>:
</p>
<div class="smallexample">
<pre class="smallexample">(gdb) info line m4_changequote
Line 895 of &quot;builtin.c&quot; starts at pc 0x634c and ends at 0x6350.
</pre></div>

<a name="index-code-address-and-its-source-line"></a>
<p>We can also inquire (using <code>*<var>addr</var></code> as the form for
<var>location</var>) what source line covers a particular address:
</p><div class="smallexample">
<pre class="smallexample">(gdb) info line *0x63ff
Line 926 of &quot;builtin.c&quot; starts at pc 0x63e4 and ends at 0x6404.
</pre></div>

<a name="index-_0024_005f-and-info-line"></a>
<a name="index-x-command_002c-default-address"></a>
<a name="index-x_0028examine_0029_002c-and-info-line"></a>
<p>After <code>info line</code>, the default address for the <code>x</code> command
is changed to the starting address of the line, so that &lsquo;<samp>x/i</samp>&rsquo; is
sufficient to begin examining the machine code (see <a href="Memory.html#Memory">Examining Memory</a>).  Also, this address is saved as the value of the
convenience variable <code>$_</code> (see <a href="Convenience-Vars.html#Convenience-Vars">Convenience
Variables</a>).
</p>
<dl compact="compact">
<dd><a name="index-disassemble"></a>
<a name="index-assembly-instructions"></a>
<a name="index-instructions_002c-assembly"></a>
<a name="index-machine-instructions"></a>
<a name="index-listing-machine-instructions"></a>
</dd>
<dt><code>disassemble</code></dt>
<dt><code>disassemble /m</code></dt>
<dt><code>disassemble /s</code></dt>
<dt><code>disassemble /r</code></dt>
<dd><p>This specialized command dumps a range of memory as machine
instructions.  It can also print mixed source+disassembly by specifying
the <code>/m</code> or <code>/s</code> modifier and print the raw instructions in hex
as well as in symbolic form by specifying the <code>/r</code> modifier.
The default memory range is the function surrounding the
program counter of the selected frame.  A single argument to this
command is a program counter value; <small>GDB</small> dumps the function
surrounding this value.  When two arguments are given, they should
be separated by a comma, possibly surrounded by whitespace.  The
arguments specify a range of addresses to dump, in one of two forms:
</p>
<dl compact="compact">
<dt><code><var>start</var>,<var>end</var></code></dt>
<dd><p>the addresses from <var>start</var> (inclusive) to <var>end</var> (exclusive)
</p></dd>
<dt><code><var>start</var>,+<var>length</var></code></dt>
<dd><p>the addresses from <var>start</var> (inclusive) to
<code><var>start</var>+<var>length</var></code> (exclusive).
</p></dd>
</dl>

<p>When 2 arguments are specified, the name of the function is also
printed (since there could be several functions in the given range).
</p>
<p>The argument(s) can be any expression yielding a numeric value, such as
&lsquo;<samp>0x32c4</samp>&rsquo;, &lsquo;<samp>&amp;main+10</samp>&rsquo; or &lsquo;<samp>$pc - 8</samp>&rsquo;.
</p>
<p>If the range of memory being disassembled contains current program counter,
the instruction at that location is shown with a <code>=&gt;</code> marker.
</p></dd>
</dl>

<p>The following example shows the disassembly of a range of addresses of
HP PA-RISC 2.0 code:
</p>
<div class="smallexample">
<pre class="smallexample">(gdb) disas 0x32c4, 0x32e4
Dump of assembler code from 0x32c4 to 0x32e4:
   0x32c4 &lt;main+204&gt;:      addil 0,dp
   0x32c8 &lt;main+208&gt;:      ldw 0x22c(sr0,r1),r26
   0x32cc &lt;main+212&gt;:      ldil 0x3000,r31
   0x32d0 &lt;main+216&gt;:      ble 0x3f8(sr4,r31)
   0x32d4 &lt;main+220&gt;:      ldo 0(r31),rp
   0x32d8 &lt;main+224&gt;:      addil -0x800,dp
   0x32dc &lt;main+228&gt;:      ldo 0x588(r1),r26
   0x32e0 &lt;main+232&gt;:      ldil 0x3000,r31
End of assembler dump.
</pre></div>

<p>Here is an example showing mixed source+assembly for Intel x86
with <code>/m</code> or <code>/s</code>, when the program is stopped just after
function prologue in a non-optimized function with no inline code.
</p>
<div class="smallexample">
<pre class="smallexample">(gdb) disas /m main
Dump of assembler code for function main:
5       {
   0x08048330 &lt;+0&gt;:    push   %ebp
   0x08048331 &lt;+1&gt;:    mov    %esp,%ebp
   0x08048333 &lt;+3&gt;:    sub    $0x8,%esp
   0x08048336 &lt;+6&gt;:    and    $0xfffffff0,%esp
   0x08048339 &lt;+9&gt;:    sub    $0x10,%esp

6         printf (&quot;Hello.\n&quot;);
=&gt; 0x0804833c &lt;+12&gt;:   movl   $0x8048440,(%esp)
   0x08048343 &lt;+19&gt;:   call   0x8048284 &lt;puts@plt&gt;

7         return 0;
8       }
   0x08048348 &lt;+24&gt;:   mov    $0x0,%eax
   0x0804834d &lt;+29&gt;:   leave
   0x0804834e &lt;+30&gt;:   ret

End of assembler dump.
</pre></div>

<p>The <code>/m</code> option is deprecated as its output is not useful when
there is either inlined code or re-ordered code.
The <code>/s</code> option is the preferred choice.
Here is an example for AMD x86-64 showing the difference between
<code>/m</code> output and <code>/s</code> output.
This example has one inline function defined in a header file,
and the code is compiled with &lsquo;<samp>-O2</samp>&rsquo; optimization.
Note how the <code>/m</code> output is missing the disassembly of
several instructions that are present in the <code>/s</code> output.
</p>
<p><samp>foo.h</samp>:
</p>
<div class="smallexample">
<pre class="smallexample">int
foo (int a)
{
  if (a &lt; 0)
    return a * 2;
  if (a == 0)
    return 1;
  return a + 10;
}
</pre></div>

<p><samp>foo.c</samp>:
</p>
<div class="smallexample">
<pre class="smallexample">#include &quot;foo.h&quot;
volatile int x, y;
int
main ()
{
  x = foo (y);
  return 0;
}
</pre></div>

<div class="smallexample">
<pre class="smallexample">(gdb) disas /m main
Dump of assembler code for function main:
5	{

6	  x = foo (y);
   0x0000000000400400 &lt;+0&gt;:	mov    0x200c2e(%rip),%eax # 0x601034 &lt;y&gt;
   0x0000000000400417 &lt;+23&gt;:	mov    %eax,0x200c13(%rip) # 0x601030 &lt;x&gt;

7	  return 0;
8	}
   0x000000000040041d &lt;+29&gt;:	xor    %eax,%eax
   0x000000000040041f &lt;+31&gt;:	retq
   0x0000000000400420 &lt;+32&gt;:	add    %eax,%eax
   0x0000000000400422 &lt;+34&gt;:	jmp    0x400417 &lt;main+23&gt;

End of assembler dump.
(gdb) disas /s main
Dump of assembler code for function main:
foo.c:
5	{
6	  x = foo (y);
   0x0000000000400400 &lt;+0&gt;:	mov    0x200c2e(%rip),%eax # 0x601034 &lt;y&gt;

foo.h:
4	  if (a &lt; 0)
   0x0000000000400406 &lt;+6&gt;:	test   %eax,%eax
   0x0000000000400408 &lt;+8&gt;:	js     0x400420 &lt;main+32&gt;

6	  if (a == 0)
7	    return 1;
8	  return a + 10;
   0x000000000040040a &lt;+10&gt;:	lea    0xa(%rax),%edx
   0x000000000040040d &lt;+13&gt;:	test   %eax,%eax
   0x000000000040040f &lt;+15&gt;:	mov    $0x1,%eax
   0x0000000000400414 &lt;+20&gt;:	cmovne %edx,%eax

foo.c:
6	  x = foo (y);
   0x0000000000400417 &lt;+23&gt;:	mov    %eax,0x200c13(%rip) # 0x601030 &lt;x&gt;

7	  return 0;
8	}
   0x000000000040041d &lt;+29&gt;:	xor    %eax,%eax
   0x000000000040041f &lt;+31&gt;:	retq

foo.h:
5	    return a * 2;
   0x0000000000400420 &lt;+32&gt;:	add    %eax,%eax
   0x0000000000400422 &lt;+34&gt;:	jmp    0x400417 &lt;main+23&gt;
End of assembler dump.
</pre></div>

<p>Here is another example showing raw instructions in hex for AMD x86-64,
</p>
<div class="smallexample">
<pre class="smallexample">(gdb) disas /r 0x400281,+10
Dump of assembler code from 0x400281 to 0x40028b:
   0x0000000000400281:  38 36  cmp    %dh,(%rsi)
   0x0000000000400283:  2d 36 34 2e 73 sub    $0x732e3436,%eax
   0x0000000000400288:  6f     outsl  %ds:(%rsi),(%dx)
   0x0000000000400289:  2e 32 00       xor    %cs:(%rax),%al
End of assembler dump.
</pre></div>

<p>Addresses cannot be specified as a location (see <a href="Specify-Location.html#Specify-Location">Specify Location</a>).
So, for example, if you want to disassemble function <code>bar</code>
in file <samp>foo.c</samp>, you must type &lsquo;<samp>disassemble 'foo.c'::bar</samp>&rsquo;
and not &lsquo;<samp>disassemble foo.c:bar</samp>&rsquo;.
</p>
<p>Some architectures have more than one commonly-used set of instruction
mnemonics or other syntax.
</p>
<p>For programs that were dynamically linked and use shared libraries,
instructions that call functions or branch to locations in the shared
libraries might show a seemingly bogus location&mdash;it&rsquo;s actually a
location of the relocation table.  On some architectures, <small>GDB</small>
might be able to resolve these to actual function names.
</p>
<dl compact="compact">
<dd><a name="index-set-disassembly_002dflavor"></a>
<a name="index-Intel-disassembly-flavor"></a>
<a name="index-AT_0026T-disassembly-flavor"></a>
</dd>
<dt><code>set disassembly-flavor <var>instruction-set</var></code></dt>
<dd><p>Select the instruction set to use when disassembling the
program via the <code>disassemble</code> or <code>x/i</code> commands.
</p>
<p>Currently this command is only defined for the Intel x86 family.  You
can set <var>instruction-set</var> to either <code>intel</code> or <code>att</code>.
The default is <code>att</code>, the AT&amp;T flavor used by default by Unix
assemblers for x86-based targets.
</p>
<a name="index-show-disassembly_002dflavor"></a>
</dd>
<dt><code>show disassembly-flavor</code></dt>
<dd><p>Show the current setting of the disassembly flavor.
</p></dd>
</dl>

<dl compact="compact">
<dd><a name="index-set-disassemble_002dnext_002dline"></a>
<a name="index-show-disassemble_002dnext_002dline"></a>
</dd>
<dt><code>set disassemble-next-line</code></dt>
<dt><code>show disassemble-next-line</code></dt>
<dd><p>Control whether or not <small>GDB</small> will disassemble the next source
line or instruction when execution stops.  If ON, <small>GDB</small> will
display disassembly of the next source line when execution of the
program being debugged stops.  This is <em>in addition</em> to
displaying the source line itself, which <small>GDB</small> always does if
possible.  If the next source line cannot be displayed for some reason
(e.g., if <small>GDB</small> cannot find the source file, or there&rsquo;s no line
info in the debug info), <small>GDB</small> will display disassembly of the
next <em>instruction</em> instead of showing the next source line.  If
AUTO, <small>GDB</small> will display disassembly of next instruction only
if the source line cannot be displayed.  This setting causes
<small>GDB</small> to display some feedback when you step through a function
with no line info or whose source file is unavailable.  The default is
OFF, which means never display the disassembly of the next line or
instruction.
</p></dd>
</dl>


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